I'm convinced the Wishlist could be turned into a much more powerful tool to drive sales and to keep players informed about their most anticipated games.

Apart from a few standard filtering options which only change the visualised order of your listed games, we currently only have a simple ranking system to actually sort our lists and showcase the order in our profile - but the reasons for which we add games to the Wishlist can't simply be ranked in numbers.

• We add games that we'd like to buy eventually, but not immediately.
  
• We add games that we really want, for friends and family to get gift ideas from.
  
• We add games that we'd check out during sales.
  
• We add games that were just announced, so we get news and maybe preorder.
  
• We add games on Early Access to wait for their launch.
  
• We add games that are newly released/EAd, but with dissatisfying ratings, and want to see if and how they progress.
  
• I would LIKE to add games to my wishlist that I already own - so that I can tell friends to whom I recommended those that they are on sale, or to buy them as a gift during sales.

What the Wishlist really needs is a function to divide the list into categories, similar to the Library's collections or shelves.

Key features:

• The process of wishlisting would remain unchanged, and add newly wishlisted games to an uncategorised part of the list.
  
• Users should be able to create, delete, name and arrange categories/shelves manually (as in, drag and drop each shelf into the desired position rather than automated alphabetical sorting. Hate the alphabetical sorting of Library shelves).
  
• Categories/shelves should be collapsible/expendable for easier viewing.
  
• Users should be able to drag and drop Wishlist items into each category and position like we currently do in the ranked Wishlist.
  
• The arranging of Wishlist items should happen manually, maybe with an option for dynamic shelves like in the Library (I mean, the rough code is already there, why not use it).
  
• This categorised Wishlist should be the one viewable by others, effectively replacing the plain ranked view we currently have.

I feel like it's a bit of an oversight on Valve's side that you can't wishlist items you own, because gifting games and suggesting games to others is a sales factor, and I often end up manually checking certain games for sales in the shop or on SteamDB to recommend a game to friends for its current price.
If I could add games that I own to my Wishlist and categorise it as such, I'd have better opportunities to suggest great games on sale to friends, or gift them.

One could even make a category for games that'd make a great gift for their own upcoming birthday, Christmas etc., so you don't end up with a gifted game that is good, but for which you're currently not hyped, while something else in the same price range would have scratched that itch.

Personally, I have about 400 games on my Wishlist right now, I loosely ranked the top 30 or so games, and the rest is a wild, uncontrollable mix of "yeah maybe, but only if the ratings improve", "might be good with this one friend group", "can't wait for this to actually leave Early Access in 2039", "if I ever get back into game X I'll clearly need all 110 DLCs" and "ehh maybe if it ever goes on a deep sale for 5 bucks".

I'd like to split co-op games that I wishlist to play with friends and family from my singleplayer games, simply because these have different triggers for buying them. When I'm looking for a new co-op game to play with someone, I'd rather look at a personally ranked list of co-op games, and not look for co-op games in my loosely ranked list of 400-600 games.

I'd also make categories for genres as well, so I'd know where to look when I want to get a new Horror/Cozy/RPG game.

If I could add Early Access games to my Wishlist in a respective category, I could check this category at any time and see how the games are doing at the moment - check ratings, prices, EA status... It'd make it easy to buy them, move them to other categories, or remove them from the wishlist.
The same benefits would apply to announced titles, for which I'd make a category to track promising games in their early stages.

---

So in summary, I think that the current Wishlist having little to no functionality hinders its sales potential, and makes it hard for avid players to keep an overview over the plethora of games on their Wishlist and the many different reasons why these were added at some point, and which trigger would convince us that this is the right time to buy.
Even a blatant copy of the Library's current system of collections and shelves would add a lot to the Wishlist's usefulness and viewability.